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United States Patent |
5,668,173
|
Garrow
|
September 16, 1997
|
Method of increasing the conversion of homocysteine to methionine and
uses thereof
Abstract
The present invention provides a method of decreasing total plasma
homocyst(e)ine levels in an animal in need of such treatment, comprising
the step of administering to said animal a pharmacologically effective
dose of a thetin. Further provided is a method of treating
hyperhomocyst(e)inemia in an animal in need of such treatment, comprising
the step of administering to said animal a pharmacologically effective
dose of a thetin. Also provided are novel pharmaceutical and nutritional
compositions. Further provided is a method of reducing or eliminating the
costly supplementation of methionine, choline, and betaine to
protein-containing animal feeds.
Inventors:
|
Garrow; Timothy A. (Champaign, IL)
|
Assignee:
|
The Board of Trustees of the University of Illinois Corp. (Urbana, IL)
|
Appl. No.:
|
605940 |
Filed:
|
February 23, 1996 |
Current U.S. Class: |
514/557 |
Intern'l Class: |
A61K 031/19 |
Field of Search: |
514/556,557,562
|
References Cited
U.S. Patent Documents
5139791 | Aug., 1992 | Nakajima et al. | 426/2.
|
Primary Examiner: Henley, III; Raymond
Attorney, Agent or Firm: Adler; Benjamin Aaron
Claims
What is claimed is:
1. A method of decreasing homocysteine levels in an animal in need of such
treatment, comprising the step of administering to said animal a
pharmacologically effective dose of a thetin.
2. The method of claim 1, wherein said animal is a human.
3. The method of claim 1, wherein said animal has hyperhomocysteinemia.
4. The method of claim 1, wherein thetin is selected from the group
consisting of dimethylacetothetin and dimethylpropriothetin.
5. The method of claim 1, wherein thetin is administered in a dose of from
about 1 mg/kg to about 50 mg/kg.
6. A method of treating hyperhomocysteinemia in an animal in need of such
treatment, comprising the step of administering to said animal a
pharmacologically effective dose of a thetin.
7. The method of claim 6, wherein said animal is a human.
8. The method of claim 6, wherein thetin is selected from the group
consisting of dimethylacetothetin and dimethylpropriothetin.
9. The method of claim 6, wherein thetin administered in a dose of from
about 1 mg/kg to about 50 mg/kg.
10. An nutritional supplement consisting essentially of a thetin selected
from the group consisting of dimethylacetothetin and dimethylpropriothetin
and at least one vitamin.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the fields of cardiovascular
pharmacology and therapeutics, and animal nutrition. More specifically,
the present invention relates to a method of decreasing plasma
homocysteine levels in animals with hyperhomocyst(e)inemia. This invention
also relates to a method of reducing or eliminating the supplemental
methionine, choline, and betaine routinely added to animal feeds.
2. Description of the Related Art
Pharmaceutical or nutriceutical applications of the invention.
Cardiovascular disease (CVD), i.e., coronary, cerebral, and peripheral
atherosclerosis and thrombosis, is the major cause of death in the United
States. The age-adjusted death rate from CVD decreased about 40 percent
from 1964 to 1985 (1). This decline was attributed to better health care
and lifestyle changes. Despite this decline, more than one fourth of all
Americans still suffer from some form of CVD, and almost 50% die of this
disease (2). The cost of CVD to Americans in direct health care
expenditures and lost productivity is estimated to be $110 billion each
year (2). This cost estimate is expected to rise since the population of
elderly in this country is increasing.
The cause of CVD is multifactorial. Some well known risk factors include
hypertension, smoking, and high blood cholesterol. Many risk factors are
influenced by genetic predisposition and diet. It has been shown that
hyperhomocyst(e)inemia is associated with the premature development of CVD
(3, 4). Furthermore, there is evidence that suggest that the relationship
between plasma homocysteine and CVD is causal and not just a marker for
another risk factor since vascular lesions have been induced in primates
by infusing homocysteine for a 3 month period. The risk of fatal
thrombosis is reduced in homocystinurics undergoing plasma
homocysteine-lowering treatment.
Fasting plasma homocysteine levels have been classified as either, normal
(10-15 .mu.M), or one of the following levels of hyperhomocyst(e)inemia;
moderate (15-30 .mu.M), intermediate (31-100 .mu.M), or severe (>100
.mu.M). Like blood cholesterol, the relationship of plasma homocysteine to
CVD appears to be graded (5). This means that any increase in plasma
homocysteine above normal is associated with an increased risk, and that
the higher the elevation, the greater the risk. Several nutritional and
genetic determinants have been identified that cause
hyperhomocyst(e)inemia and some of these determinants are more common than
previously imagined. The mechanistic details of how elevations in plasma
homocysteine promote CVD is not completely understood.
Epidemiological reports indicate that 15-40% of CVD patients have elevated
levels of plasma homocysteine (6). Studies have shown associations of
plasma homocysteine with age, sex, smoking, hypertension, and total serum
cholesterol (3, 4, 7). Plasma homocysteine increases with age and is
higher in males than females. There is a positive linear association
between plasma homocysteine and serum cholesterol levels. Both smoking and
hypertension have multiplicative effects on plasma homocysteine (7).
The association of hyperhomocyst(e)inemia with some of the most potent risk
factors for CVD, such as smoking, hypertension, and cholesterol
metabolism, makes managing the levels of plasma homocysteine an important
public health goal. An effective dietary or pharmaceutical treatment for
hyperhomocyst(e)inemia would be expected to decrease the mortality and
morbidity from CVD and result in considerable health care savings.
One of the most effective treatments for severe hyperhomocyst(e)inemia is
the oral administration of pharmacological doses of betaine, either alone,
or concurrent with vitamin supplementation. Betaine, a metabolite of
choline oxidation, is a substrate for an enzyme called
betaine-homocysteine methyltransferase. Betaine-homocysteine
methyltransferase catalyzes the conversion of betaine and homocysteine to
dimethylglycine and methionine, respectively. The treatment of
hyperhomocyst(e)inemia with betaine reduces plasma homocysteine by
increasing the conversion of homocysteine to methionine via the
betaine-homocysteine methyltransferase catalyzed reaction. Normal levels
of plasma homocysteine, however, are rarely attained by this treatment
leaving considerable CVD risk for these individuals.
Animal feed applications relating to methionine. The animal feed industry
typically formulates protein-containing organic feeds for domestic animal
consumption. These feeds are often supplemented with various nutrients so
that they meet the specific dietary requirements of a given animal species
and therefore improve some measure of animal performance. Animal
performance being defined herein as including but not limited to the
physiological states of growth, gestation, and lactation.
Many feeds contain corn or soybean meal, or a mixture of these two
feedstuffs, as a base ingredient. These feeds are routinely supplemented
with methionine and choline to meet the recommended dietary intakes of
these compounds (8-10). For example, methionine is typically added to
these practical diets because they are often deficient in this essential
amino acid. Choline also is added to these feeds. Although most animals do
not have an absolute requirement for choline, and one notable exception is
the chicken, choline is routinely added to animal feeds because it has
methionine-sparing effects, that is, it reduces an animals dietary
requirement for methionine. Betaine, a metabolite of choline oxidation,
also is added to feeds for methionine- and choline-sparing effects. Some
studies suggest that dietary betaine can reduce carcass fat in pigs and
chickens and is effective in the treatment of diarrhea and wet litter in
fowl.
In summary, the prior art is deficient in the lack of an effective methods
to decrease plasma homocysteine levels in animals with
hyperhomocyst(e)inemia. In addition, the prior art is deficient in a
method to maximize the conversion of homocysteine to methionine while
simultaneously decreasing the level of dietary methionine, choline and
betaine added to animal feeds necessary to obtain optimal animal
performance. The present invention fulfills this long-standing need and
desire in the art.
SUMMARY OF THE INVENTION
Alternate methyl donor substrates for betaine-homocysteine
methyltransferase, described herein, are more effective plasma
homocysteine-lowering agents for the treatment of hyperhomocyst(e)inemia.
This invention eliminates or reduces the supplemental methionine, choline,
and betaine that is routinely added to practical animal feeds. This
invention results in significant reductions in the cost of maintaining
optimal animal performance.
In one embodiment of the present invention, there is provided a method of
decreasing plasma homocysteine levels in an animal in need of such
treatment, comprising the step of administering to said animal a
pharmacologically effective dose of a thetin.
In another embodiment of the present invention, there is provided a
pharmaceutical composition, comprising a thetin and a pharmaceutically
acceptable carrier.
In another embodiment of the present invention, there is provided an animal
feed supplement consisting essentially of a thetin selected from the group
consisting of dimethylacetothetin and dimethylpropiothetin to lower the
dietary requirement, and therefore feed supplement, of methionine, choline
and betaine.
In yet another embodiment of the present invention, there is provided a
method of treating hyperhomocysteinemia in an animal in need of such
treatment, comprising the step of administering to said animal a
pharmacologically effective dose of a thetin.
In yet another embodiment of the present invention, there is provided a
nutritional supplement consisting essentially of a thetin selected from
the group consisting of dimethylacetothetin and dimethylpropiothetin and
at least one vitamin. The nutritional supplement may be taken either
alone, or in combination with other dietary supplements.
In yet another embodiment of the present invention, there is provided a
composition consisting essentially of a thetin selected from the group
consisting of dimethylacetothetin and dimethylpropiothetin and choline. A
further composition described herein consists essentially of a thetin
selected from the group consisting of dimethylacetothetin and
dimethylpropiothetin and betaine.
Other and further aspects, features, and advantages of the present
invention will be apparent from the following description of the presently
preferred embodiments of the invention given for the purpose of disclosure
.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the matter in which the above-recited features, advantages and
objects of the invention, as well as others which will become clear, are
attained and can be understood in detail, more particular descriptions of
the invention briefly summarized above may be had by reference to certain
embodiments thereof which are illustrated in the appended drawings. These
drawings form a part of the specification. It is to be noted, however,
that the appended drawings illustrate preferred embodiments of the
invention and therefore are not to be considered limiting in their scope.
FIG. 1 shows an outline of homocysteine and methionine metabolism.
Abbreviations: THF, tetrahydrofolate; SAM, S-adenosylmethionine; SAH,
S-adenosylhomocysteine; HCY, homocysteine; ser, serine; cys, cysteine.
Reactions: 1, serine hydroxymethyltransferase (vitamin B.sub.6
-dependent); 2, methylene-THF reductase; 3, methionine synthase (B.sub.12
-dependent); 4, betaine-homocysteine methyltransferase; 5,
cystathionine-.beta.-synthase (vitamin B.sub.6 -dependent); 6,
cystathionase. Reactions 5 and 6 are referred to as the transsulfuration
pathway. Deficiencies in reactions 2 and 5 cause hyperhomocyst(e)inemia in
humans. Inborn errors of cobalamin metabolism also can cause
hyperhomocyst(e)inemia in humans due to reduced availability of methyl
cobalamin (B.sub.12)for reaction 3.
FIG. 2 shows the only known homocysteine-independent (transamination)
pathway for methionine catabolism. Note that 3-methylthioproprionate is a
proposed intermediate in this pathway. 3-Methylthiopropionate is a product
of the betaine-homocysteine methyltransferase catalyzed reaction when
dimethylpropriothetin is the methyl-donating substrate. Structures of
3-methylthioproprionate and dimethylpropriothetin can be seen in FIG. 3.
FIG. 3 shows the betaine-homocysteine methyltransferase catalyzed reaction.
Betaine-homocysteine methyltransferase is an enzyme in the choline
oxidation pathway. FIG. 3A shows the reaction with betaine, a metabolite
of choline oxidation. FIG. 3B shows alternate (thetin) methyl
donor-product pairs for the betaine-homocysteine methyltransferase
catalyzed reaction.
FIG. 4 shows the biochemical pathway of choline oxidation. Abbreviations:
HCY, homocysteine; MET, methionine; THE, tetrahydrofolate; CH2THF,
methylenetetrahydrofolate. Reactions: 1, choline dehydrogenase; 2, betaine
aldehyde dehydrogenase; 3, betaine-homocysteine methyltransferase; 4,
dimethylglycine dehydrogenase; 5, sarcosine dehydrogenase; 6, glycine
cleavage. The oxidation of choline takes place primarily in the liver and
kidney.
FIG. 5 shows sodium dodecylsulfate polyacrylamide electrophoretic analysis
of pig liver betaine-homocysteine methyltransferase. Lane 1, MW standards.
Lane 2, DEAE cellulose purified enzyme.
FIG. 6 shows a Lineweaver-Burke plot of initial rate velocities of pig
liver betaine-homocysteine methyltransferase activity using subsaturating
concentrations of betaine (fixed: 25, 50, 100 .mu.M) and homocysteine
(variable). The converging family of lines indicate that the kinetic
mechanism is sequential.
FIG. 7 shows the inhibition (Dixon plots) of purified pig liver
betaine-homocysteine methyltransferase activity by dimethylglycine.
Betaine was fixed at 25 .mu.M (panel A) or 250 .mu.M (panel B) while
L-homocysteine was varied. Parallel lines indicate uncompetitive
inhibition relative to homocysteine.
DETAILED DESCRIPTION OF THE INVENTION
The following abbreviations and definitions are used herein: The term "CVD"
refers to cardiovascular disease. The term "plasma homocysteine" denotes
total plasma homocyst(e)ine. The term "hyperhomocyst(e)inemia" denotes any
level of total plasma homocyst(e)ine above normal.
Homocysteine metabolism
Homocysteine and methionine metabolism are related since methionine is the
ultimate source of homocysteine in the body. An outline of homocysteine
and methionine metabolism can be seen in FIG. 1. Methionine is an
essential nutrient for many animals, and like all amino acids, is required
for protein synthesis. Another role for methionine, however, depends upon
its conversion to S-adenosylmethionine, an important compound used for
many biological methylation reactions. When used in various
methyltransferase reactions, S-adenosylmethionine is converted to
S-adenosylhomocysteine, which in turn is hydrolyzed to produce
homocysteine. Homocysteine lies at a metabolic branch point, it can be
methylated to regenerate methionine by either methionine synthase
(reaction 3, FIG. 1 ) or betaine-homocysteine methyltransferase (reaction
4, FIG. 1), or it can be metabolized to cysteine through the
transsulfuration pathway. Transsulfuration is a catabolic pathway for
methionine and begins with the physiologically irreversible condensation
of serine and homocysteine by cystathionine-.beta.-synthase (reaction 5,
FIG. 1).
An alternate pathway (transamination) for methionine catabolism, that does
not proceed through a homocysteine intermediate, has been described (11,
12), however, all of the enzymatic steps in this pathway have not been
elucidated. This pathway is outlined in FIG. 2 and begins with a
transamination step to form the .alpha.-keto acid of methionine,
.alpha.-keto-.gamma.-methiolbutyrate. This .alpha.-keto acid is then
decarboxylated by .alpha.-keto-.gamma.-methiolbutyrate decarboxylase to
form 3-methylthiopropionate. This decarboxylase catalyzes the
rate-limiting reaction in this pathway. Subsequent steps have not been
fully characterized and the quantitative significance of this pathway to
methionine catabolism, under normal or pathophysiological states, is
unknown.
The factors that regulate homocysteine metabolism are poorly understood. It
has been reported that humans consuming normal diets methylate
approximately 50% of the available homocysteine while the remaining half
proceeds through the transsulfuration pathway (13). However, the dietary
intake of methionine influences the rate of transulfuration and
methylation of homocysteine in rat liver. When dietary protein is in
excess approximately 70% of the homocysteine is converted to cysteine and
the labile methyl carbon on methionine turns over twice as fast (14). On
low protein diets, only 10% of the available homocysteine proceeds through
the transsulfuration pathway (14, 15). S-adenosylmethionine is an
important modulator of these pathways since it is an allosteric activator
(16) of cystathionine-b-synthase and a potent inhibitor (17) of
methylenetetrahydrofolate reductase (refer to FIG. 1). Furthermore, a
recent study shows that cystathionine-.beta.-synthase activity and steady
state levels of its mRNA decrease when dietary cysteine is increased (18).
With regard to the methylation of homocysteine, in vitro simulation studies
indicate that methionine synthase and betaine-homocysteine
methyltransferase contribute equally to this process (15). However, other
studies have shown that methionine synthase and betaine-homocysteine
methyltransferase activities change in response to various nutritional and
hormonal treatments when measured in crude liver extract. Thus, the
relative importance of one methyltransferase to the other may change
depending upon physiological state (19, 20).
Hyperhomocyst(e)inemia: known causes and current treatments.
Nutritional and genetic factors have been identified that cause some degree
of hyperhomocyst(e)inemia in humans (FIG. 1). Nutrient deficiencies of
either folate, vitamin B6, or vitamin B12, can result in moderate to
intermediate hyperhomocyst(e)inemia in humans. Hyperhomocyst(e)inemia also
can result from genetically determined deficiencies of key enzymes in
homocysteine metabolism, namely, cystathionine-.beta.-synthase or
methylenetetrahydrofolate reductase activities, or inborn errors in the
metabolism of vitamin B12 (cbl mutations). Cbl mutations cause
hyperhomocyst(e)inemia by reducing the activity of vitamin B12-dependent
methionine synthase. These genetic defects cause moderate to severe
hyperhomocyst(e)inemia depending upon the nature of the mutation and
whether both alleles are afflicted.
Nutritional therapies for hyperhomocyst(e)inemia have been implemented with
some success. Individuals are usually supplemented with either folio acid,
vitamin B12, vitamin B6, or a combination of these vitamins. Dietary
methionine also has been restricted in cystathionine-.beta.-synthase
deficient patients. Individuals who are vitamin deficient, or who have
genetic mutations that affect coenzyme affinity, respond favorably to this
treatment. For example, approximately 50% of individuals with
cystathionine-.beta.-synthase deficiency respond to supplemental B6
whereas the remaining 50% are vitamin B6 non-responsive. Some individuals
with severe forms of cystathionine-.beta.-synthase or
methylenetetrahydrofolate reductase deficiency who have severe
hyperhomocyst(e)inemia, do not respond to vitamin therapy. Pharmacological
doses of betaine have been shown to reduce plasma homocysteine in these
individuals (21-24). Betaine lowers plasma homocysteine by increasing the
flux through betaine-homocysteine methyltransferase but this treatment
does not usually lower plasma homocysteine to within the normal range, and
the moderate to intermediate hyperhomocyst(e)inemia that persists confer
considerable CVD risk for these individuals.
The betaine-homocysteine methyltransferase catalyzed reaction
Betaine-homocysteine methyltransferase catalyzes the conversion of betaine
and homocysteine to dimethylglycine and methionine, respectively (FIG. 3).
This reaction is required for the oxidation of choline (FIG. 4).
Betaine-homocysteine methyltransferase is primarily found in the liver and
kidney of animals (25-27). Betaine-homocysteine methyltransferase has been
purified from rat (28, 29), horse (30), human (31), and pig liver (32), an
organ that expresses very high levels of this enzyme. Betaine-homocysteine
methyltransferase is a hexamer of identical subunits of approximately 45
kDa.
Betaine-homocysteine methyltransferase activity is modulated by nutritional
status. Finkelstein has reported higher levels of betaine-homocysteine
methyltransferase activity in crude liver extracts when rats consume
surfeit levels of dietary choline, betaine, or methionine compared to
control animals (33, 34). In addition, methionine deficiency also elevates
hepatic betaine-homocysteine methyltransferase activity. Similar effects
of dietary methionine, choline, and betaine intakes on hepatic
betaine-homocysteine methyltransferase activity have been observed in
chickens. These findings suggest that betaine-homocysteine
methyltransferase functions to conserve homocysteine when dietary
methionine is low, and metabolize excess betaine. Excess betaine can be
provided either preformed in the diet or produced from excess dietary
choline. Betaine-homocysteine methyltransferase activity also has been
reported to be influenced by hormonal status.
Kinetic studies of the betaine-homocysteine methyltransferase catalyzed
reaction have been limited. There are several reports describing
alternative substrates as methyl donors, and the ability of products, or
substrate and product analogs, to inhibit the betaine-homocysteine
methyltransferase reaction (25, 29, 31, 35, 36, 37). The Michealis
constants for betaine and homocysteine have been estimated using rat and
human betaine-homocysteine methyltransferase containing extracts and are
reportedly in the 25-50 .mu.M range.
The kinetic mechanism of betaine-homocysteine methyltransferase has been
evaluated using partially purified rat enzyme (28, 38). Both studies
concluded that the reaction mechanism was sequential. One study suggested
that the reaction is ordered with homocysteine being the first substrate
to bind, and methionine being the last product to come off the enzyme,
however, data were not presented to substantiate their conclusions.
Thetins are alternate substrates for the betaine-homocysteine
methyltransferase reaction and increase the conversion of homocysteine to
methionine
Several studies have shown that dimethylacetothetin and
dimethylpropiothetin are substrates for betaine-homocysteine
methyltransferase (FIG. 3). These studies assayed betaine-homocysteine
methyltransferase activity using saturating levels of substrates, and in
each case, after correcting for different enzyme concentrations, more
methionine was produced with thetins as methyl donors than when betaine
was used. Prior studies have shown that these thetins can replace dietary
choline and betaine and are non-toxic to animals. Furthermore,
dimethylpropiothetin is a compound that is synthesized by some
salt-tolerant plants and marine animals.
Initial rate studies of the homocysteine-dependent betaine-homocysteine
methyltransferase catalyzed demethylation of dimethylacetothetin can be
found in Example I below. These studies clearly show that
dimethylacetothetin displays greater specificity (Vmax/Km) for the
betaine-homocysteine methyltransferase reaction than betaine. The product
of the dimethylacetothetin-dependent reaction has lower affinity to
betaine-homocysteine methyltransferase than does the product of the
betaine-dependent reaction. The literature cited above, and the studies
reported below, indicate that thetins will increase the rate of methionine
formation from homocysteine and will be effective as plasma
homocysteine-lowering agents and as feed additives with methionine-,
choline-, and betaine-sparing effects.
The present invention is directed to a method of decreasing plasma
homocysteine levels in an animal in need of such treatment, comprising the
step of administering to said animal a pharmacologically effective dose of
a thetin compound or biochemically similar analogue thereof. Preferably,
the methods of the present invention will be useful in humans. Even more
preferably, a method of decreasing homocysteine levels in an animal who
has hyperhomocyst(e)inemia will be useful.
Generally, any thetin which decreases or inhibits the physiological levels
of homocysteine in the animal will be useful in the methods of the present
invention Preferably, the thetin is selected from the group consisting of
dimethylacetothetin and dimethylpropriothetin, and any salts or analogues
thereof. Specifically contemplated analogues include ester analogues of
dimethylacetothetin and dimethylpropriothetin, i.e, methyl ester, ethyl
ester, etc.
Generally, the thetin compounds may be administered in the methods of the
present invention in any concentration which decreases physiological
levels of homocysteine in the animal. Preferably, the thetin is
administered in a dose of from about 1 mg mg/kg to about 50 mg/kg. Betaine
and thetins are of approximately the same molecular weight. The
pharmacological doses of betaine given to patients with severe
hyperhomocyst(e)inemia is 3-6 grams/day. If average person is 50-70 kg,
e.g., 60 kg, then they are getting approximately 50-100 mg/kg body weight
per day. Treatment for more mild forms of hyperhomocyst(e)inemia or other
related pathologies elevating plasma homocysteine would likely require
less. Furthermore, a human on average consumes 500 g of food per day (dry
weight basis). Then 5 g would be about 1% of the diet. For animal feed
supplements, up to about 0.75% of the diet on a dry weight basis would be
useful.
The present invention is also directed to a pharmaceutical composition,
comprising a thetin and a pharmaceutically acceptable carrier. In the
pharmaceutical composition, the thetin is preferably selected from the
group consisting of dimethylacetothetin and dimethylpropriothetin, and any
salts or analogues thereof.
The present invention is also directed to an animal feed supplement
consisting essentially of a thetin selected from the group consisting of
dimethylacetothetin and dimethylpropriothetin or any salts or analogues
thereof as a means of decreasing the feed levels of methionine, choline,
and/or betaine while maintaining optimal animal performance. This
invention is applicable to the diet of animals, which herein is defined as
including fowl and mammals.
The present invention is also directed to a method of treating
hyperhomocyst(e)inemia in an animal in need of such treatment, comprising
the step of administering to said animal a pharmacologically effective
dose of a thetin.
It is specifically contemplated that the present invention may be used in
combination with more conventional therapies. For example, thetin(s) may
be used in combination with betaine, choline, and/or vitamins and other
nutritional therapies. Thus, the present invention also is directed to a
nutritional supplement consisting essentially of a thetin selected from
the group consisting of dimethylacetothetin and dimethylpropriothetin and
at least one vitamin. The nutritional supplement may be taken either
alone, or in combination with other dietary supplements.
In yet another embodiment of the present invention, there is provided a
composition consisting essentially of a thetin selected from the group
consisting of dimethylacetothetin and dimethylpropiothetin and choline. A
further composition described herein consists essentially of a thetin
selected from the group consisting of dimethylacetothetin and
dimethylpropiothetin and betaine. Concentrations of choline and betaine
useful in these compositions are well known to those having ordinary skill
in the art.
The following examples are given for the purpose of illustrating various
embodiments of the invention and are not meant to limit the present
invention in any fashion.
EXAMPLE 1
Purification of pig liver betaine-homocysteine methyltransferase and enzyme
kinetics: comparison of betaine and dimethylacetothetin as methyl donors
Betaine was prepared and purified by procedures that have been previously
described (39, 40). The chloride salts of dimethylacetothetin and
dimethylpropiothetin were synthesized by two general methods (41, 42). In
brief, one approach used equimolar amounts of methylsulfide, and either
iodo- or chloro-acetic acid for dimethylacetothetin synthesis, or
iodopropionic acid for dimethylpropiothetin synthesis. Ethyl alcohol or
ether were used as reaction solvents. A second approach used equal molar
concentrations of methyl iodide and either methylthioacetic, or
methylthioproprionic acid, in aqueous formic acid. When necessary,
treatment with silver chloride, and subsequent removal of silver iodide by
filtration, was performed to make the chloride salts. Following filtration
or evaporation steps, thetins were purified by recrystallization from hot
ethanol with ether. Structures and purities were verified by proton
nuclear magnetic resonance. Radiolabeled (.sup.14 C) dimethylacetothetin
chloride was prepared by reacting methylsulfide with .sup.14 C-labeled
iodoacetic acid.
Betaine-homocysteine methyltransferase assay
Betaine-homocysteine methyltransferase activity was measured as described
by Finkelstein and Mudd (43) with the following minor modifications.
Routine measurements used 5 mM D,L-homocysteine and 2 mM betaine (0.05-0.1
.mu.Ci), respectively, in a final reaction volume of 0.5 mL. Following a 2
hour incubation, samples were chilled to 0.degree. C. and 2.5 mL ice-cold
water was added. The samples were loaded onto a 0.5 cm (diameter) column
containing 1 mL of Dowex 1-X4 (OH--) resin (100-200 mesh). The unreacted
betaine (or dimethylacetothetin) was washed from the column with water
(3.times.5 mL), and dimethylglycine (or methylthioacetate) and methionine
eluted into scintiallation vials with 3 mL 1.5 N HCl. Blanks contained all
of the reaction components except enzyme and their values were subtracted
from the sample values. All samples were assayed in duplicate.
Michealis and Vmax constants were estimated from plotting initial rate data
according to the method of Hanes (44). Kinetic assays used L-homocysteine
instead of D,L-homocysteine. Substrate concentrations were varied where
appropriate at fixed levels of either betaine (250 .mu.M) or
L-homocysteine (500 .mu.M).
Enzyme Purification
Betaine-homocysteine methyltransferase has been purified to homogeneity
from pig liver (45). A typical purification and sodium dodecylsulfate
polyacrylamide electrophoretic analysis of a purified fraction can be seen
in TABLE I and FIG. 8, respectively.
TABLE I
______________________________________
Purification of pig liver betaine-homocysteine methyltransferase
activity* x-fold yield
fraction (units/mg) purification
(%)
______________________________________
crude 14.4 1 100
heat treated
38.7 3 96
hydroxylapatite
552 38 58
phenyl sepharose
938 65 8
DEAE cellulose
1367 95 6
______________________________________
*units are nmole methionine formed per hour. kcat .about.0.02/sec.
Using purified enzyme, initial rate studies were performed to determine the
Michaelis constants of betaine, dimethylacetothetin, and homocysteine and
to investigate the kinetic mechanism of the enzyme. Using saturating
levels of one substrate while varying the other, Michaelis constants were
estimated to be 23, 32, and 155 .mu.M for betaine, L-homocysteine, and
dimethylacetothetin, respectively. As can be seen in TABLE II, the
relative Vmax obtained using dimethylacetothetin was 47-fold greater than
when betaine was used for the methylation reaction. The Vmax/Km using
dimethylacetothetin was 7-fold greater than that for betaine indicating
that this methyl donor has greater specificity for betaine-homocysteine
methyltransferase. Thus, when present at equimolar concentrations as
betaine, the dimethylacetothetin-dependent methylation of homocysteine
would proceed at least 700% faster.
TABLE II
______________________________________
Substrate Km(.mu.M) Vmax(rel).dagger.
Vmax/Km(rel)
______________________________________
betaine 23 1 1
dimethyl 155 7 47
acetothentin
______________________________________
.dagger.Relative indicates maximum velocity corrected for an equivalent
amount of enzyme.
Dimethylacetothetin and dimethylpropiothetin have previously been shown to
be substrates for betaine-homocysteine methyltransferase (25, 28, 30, 36).
These prior studies assayed betaine-homocysteine methyltransferase using
saturating levels of either betaine or thetin, (millimolar concentrations
whereas the K.sub.m for betaine is .about.20-50 .mu.M). Depending on the
source of betaine-homocysteine methyltransferase, significant increases in
methionine production were observed. For example, when
dimethylpropiothetin or dimethylacetothetin were assayed as methyl donors
using purified horse betaine-homocysteine methyltrasferase, 54- and
350-fold increases in methionine production were observed when compared to
betaine (30). These estimates would be analogous to Vmax comparisons since
no substrate inhibition has been detected. The significantly higher Vmax
observed for the dimethylacetothetin-dependent reaction relative to the
betaine-dependent reaction reported here (TABLE II) are consistent with
previous reports.
Initial velocity data using subsaturating levels of both betaine and
homocysteine produced a family of double reciprocal plots that were linear
when homocysteine was varied at different fixed concentrations of betaine.
These lines converge at a positive 1/velocity and negative 1/substrate
value indicating that the enzyme reaction is sequential (46). This
mechanism precludes the formation of product until a ternary complex of
substrates and enzyme is formed. Sequential kinetics also have been
reported for rat betaine-homocysteine methyltransferase.
Product inhibition studies indicate that dimethylglycine is a potent
inhibitor of pig betaine-homocysteine methyltransferase (FIG. 5).
Methylthioacetate, the product of the dimethylacetothetin-dependent
reaction, has lower affinity for betaine-homocysteine methyltransferase
than dimethylglycine (TABLE III). The significantly higher Vmax of the
dimethylacetothetin-dependent reaction relative to betaine is likely due,
in part, to the reduced affinity of methylthioacetate for
betaine-homocysteine methyltransferase relative to dimethylglycine. The
subsequent metabolism of these products could have significant effects on
flux through the betaine-homocysteine methyltransferase reaction because
the rate of product removal would determine whether a product accumulates
enough to inhibit enzyme activity. When pharmacological doses of betaine
is used for the treatment of severe hyperhomocyst(e)inemia,
dimethylglycine oxidation appears to be insufficient, and as a result,
dimethylglycine accumulates.
TABLE III
______________________________________
Inhibition pf pig liver betaine-homocysteine methyltransferase
activity by dimethyglycine ethylthoacetate in the presence of
subsaturating levels of L-homocysteine (50 .mu.M) and betaine (25
.mu.M).
inhibitor (50 .mu.M)
pmol methionine
% activity
______________________________________
none 700 100
dimethylglycine
208 30
methylthioacetate
416 59
______________________________________
Although the initial rate kinetics for the dimethylpropiothetin-dependent
reaction have not been evaluated, this reaction will likely have a lower
Vmax/Km than the dimethylacetothetin-dependent reaction. Although previous
studies indicate that the dimethylpropiothetin-dependent methyl transfer
has a higher Vmax than the betaine-dependent reaction, in every case it
has been shown to have a lower Vmax than the dimethylacetothetin-dependent
reaction.
It is an object of the present invention to define the kinetic scheme for
pig liver betaine-homocysteine methyltransferase and determine the kinetic
constants of dimethylpropiothetin for the betaine-homocysteine
methyltransferase catalyzed reaction.
EXAMPLE 2
Betaine is only partially effective for the treatment of
hyperhomocyst(e)inemia
Although treating severe hyperhomocyst(e)inemia with betaine generally
lowers plasma homocysteine compared to pretreatment levels, plasma
homocysteine usually does not fall within the normal range. Since the
relationship between CVD and plasma homocysteine is graded, the inability
of betaine treatment to completely normalize plasma homocysteine leaves
significant CVD risk for these individuals (35, 47). The inability of
betaine treatment to normalize plasma homocysteine is likely related to
the kinetic properties of the betaine-homocysteine methyltransferase
reaction.
It has been shown that concomitant with a decrease in plasma homocysteine
during betaine treatment, there is also a dramatic increase in plasma (35)
and urinary (48) betaine and dimethylglycine. Increases in plasma
concentrations of betaine and dimethylglycine have been reported to be 50
to 200-fold and 40 to 125-fold, respectively. These metabolites are
usually not detected in urine. The accumulation of betaine and
dimethylglycine is likely related to the fact that the betaine-dependent
reaction has a very low Kcat that is, at least in part, due to the high
affinity dimethylglycine has for the enzyme (29, 31 and table 1).
Dimethylglycine has been reported to be a potent inhibitor of human (31,
35), and rat (38) betaine-homocysteine methyltransferases. Herein,
dimethylglycine was an uncompetitive inhibitor of pig betaine-homocysteine
methyltransferase activity when homocysteine concentrations were varied at
either subsaturating (25 .mu.M), or saturating (250 .mu.M) concentrations
of betaine (FIG. 5). It is possible that homocysteine, dimethylglycine,
and betaine-homocysteine methyltransferase form an abortive ternary
complex. Allen et al. (35) showed that methionine production could be
inhibited 50% by the addition of 10 .mu.M dimethylglycine in their assays
of human betaine-homocysteine methyltransferase activity. These kinetic
properties, i.e., a substrate conferring a very low Kcat and whose product
is a potent inhibitor, explain why high levels of betaine and
dimethylglycine are found in the urine and plasma of
hyperhomocyst(e)inemics treated with betaine and why elevated levels of
plasma homocysteine persist.
Thetins may be a more effective plasma homocysteine-lowering agents than
betaine because they are more specific substrates for betaine-homocysteine
methyltransferase and produce products with lower affinities for the
enzyme (Example I). Hence, thetins may produce a greater flux through the
betaine-homocysteine methyltransferase reaction in vivo resulting in
greater reductions in plasma homocysteine than those obtained with
betaine.
EXAMPLE 3
Use of Dimethylacetothetin and dimethylpropiothetin in vivo
Dimethylacetothetin and dimethylpropriothetin, like betaine, can replace
the dietary requirement for choline in growing rats whose diets lack
methionine but contain homocystine (41, 49, 50). Furthermore, thetin
consumption was not reported to have any toxic effects. In one study
thetins were fed at 0.5-0.93% (w/w) of the diet for 3 to 4 weeks. Food
intake and growth were not different among experimental groups consuming
either thetin, choline, or betaine as methyl donors. Furthermore, there
were no indications of kidney hemorrhage or fatty liver in animals fed any
of the methyl donors, including thetins, compared to control animals whose
diets lacked any methyl donor. Kidney hemorrhage and fatty liver are
classical signs of choline deficiency in rats. In summary, thetins can
replace dietary choline or betaine in rodents.
There is considerable data showing that the first step in the metabolism of
dimethylacetothetin and dimethylpropiothetin is through the
betaine-homocysteine methyltransferase reaction. Subsequent metabolism of
the demethylated product of the dimethylpropiothetin-dependent reaction,
3-methylthiopropionate, is through the transamination pathway of
methionine catabolism, where it enters below the rate-limiting step. The
metabolic fate of demethylated product of the
dimethylacetothetin-dependent reaction, methylthioacetate, is not
completely understood. When rats were given 60 mg of dimethylacetothetin
by either diet or subcutaneous injection, 60% of the sulfur from this
compound was found as sulfate in the urine when measured after 24 hours
(51). Therefore, thetins can be completely oxidized in mammals.
Methionine is the most toxic of all amino acids required for protein
synthesis and its toxicity has been well documented. Very high levels of
dietary 3-methylthiopropionate, an intermedate in methionine and
dimethylpropiothetin catabolism, produces toxic symptoms identical to
those produced by excess methionine consumption. The level of
3-methylthiopropionate that could be produced from the catabolism of
dimethylpropiothetin, at levels referred to herein, are well below any
level that would be toxic (52).
EXAMPLE 4
Thetins for the treatment of hyperhomocyst(e)inemia
Thetins may be more effective at reducing plasma homocysteine for the
treatment of hyperhomocyst(e)inemia than betaine because they are more
specific substrates for betaine-homocysteine methyltransferase, their
products are weaker inhibitors of betaine-homocysteine methyltransferase
than dimethylglycine (see Example I), and they are non-toxic to mammals at
levels that would be predicted to be efficacious (see Example II).
It is an object of the present invention to demonstrate the efficacy of
dietary thetins as plasma homocysteine-lowering agents using two different
rodent models for hyperhomocyst(e)inemia. The folate-depleted rat develops
intermediate hyperhomocyst(e)inemia, and cystathionine-.beta.-synthase
deficient transgenic mice (homozygous mutant) which develop severe
hyperhomocyst(e)inemia. Normal levels of plasma homocysteine in rodents
are similar to those of humans. These studies demonstrate the efficacy of
thetins relative to betaine.
EXAMPLE 5
Dimethylpropiothetin is found in nature and has osmolyte functions
Thetins are not entirely foreign to nature. Dimethylpropriothetin has been
isolated from red algae (53), two family of angiosperms (Gramineae and
Compositae; 54), two species of fish (cod and mackerel; 55 and 56), and
shellfish (57). Thus, dimethylpropiothetin is already present in the human
diet. How much of this compound is consumed by humans in not known and is
likely to vary significantly due to cultural variations in diet.
Dimethylacetothetin and dimethylpropiothetin were chemically synthesized
prior to the discovery of dimethylpropiothetin in nature.
Betaine and dimethylpropiothetin share another related function in living
cells. In animal cells, betaine accumulates to very high levels when
exposed to environments of high osmolality. Betaine and other compounds
that accumulate in such conditions are called osmolytes. These osmolytes
allow cells to retain water in environments were the extracellular
concentrations of solutes are high. In animals, osmolytes accumulate in
the renal medulla. Other compounds that have osmolyte functions in the
renal medulla are sorbitol, inositol, and taurine. In some higher plants
and marine species dimethylpropiothetin functions as an osmolyte.
Bacteria also accumulate osmolytes when grown in hyperosmotic media. For
example, Escherichia coli can convert choline to betaine, or accumulate
betaine from the environment directly, when grown in environments of high
osmolality. It has also been shown that dimethylacetothetin is equally
effective as betaine as an osmolyte in Escherichia coli.
Betaine has been given to fowl as a means to control diarrhea and wet
litter and presumably functions in this capacity as a compatible osmolyte.
Dimethylacetothetin and dimethylpropiothetin would function in a similar
manner.
EXAMPLE 6
Use of thetins to lower plasma homocysteine or increase methionine
synthesis in various clinical situations
There are other conditions where thetins may be of clinical value, either
because of a need to decrease plasma homocysteine, or increase methionine
production. Such conditions include but are not limited to, alcoholism and
liver cirrhosis, end-stage renal disease, thyroid disease, antifolate
treatment, schizophrenia, and the prevention of neural tube defects.
EXAMPLE 7
Thetins as animal feed additives
Dimethylacetothetin and dimethylpropriothetin, like betaine, have been
shown to replace the dietary requirement for choline (or betaine) in
growing rats whose diets lack methionine but contain homocystine (41, 49,
50). Therefore, thetins can reduce or eliminate the addition of choline
and betaine added to animal feeds while maintaining a similar level of
animal performance. These compounds can be made by very simple chemical
methods and therefore may improve the cost effectiveness of animal
production.
Methionine is an essential amino acid that is limiting in most practical
animal feeds to support optimal animal performance. Hence, methionine is
routinely added to animal feeds. Nutrition studies have shown that even
when sulfur amino acid intake is optimal, only about 70% of the methionine
absorbed through the intestine is retained by the animal. This indicates
that about 30% of absorbed methionine is either converted to cysteine or
completely oxidized. Thetins may be able to reduce the level of methionine
that is supplemented to protein-containing animal feeds due to enhanced
remethylation of homocysteine to form methionine by the
betaine-homocysteine methyltransferase catalyzed reaction. Increasing the
efficiency of homocysteine methylation to form methionine would decrease
the irreversible oxidation of homocysteine and therefore have a
methionine-sparing effect. This would be predicted to increase growth
rates and other measures of animal performance while reducing the need for
costly methionine supplementation of these feeds.
As a final note, dietary sulfate has been shown to decrease cystine
requirements in chickens. Dietary thetins are oxidized to sulfate, and so
would be predicted to be cystine-sparing. Practical diets for
agriculturally important animals, however, generally contain plethoric
levels of sulfate such that thetins would not be predicted to be
cystine-sparing in these diets.
It is an object of the present invention to demonstrate the efficacy of
dietary thetins as feed additives having methionine-, choline- and
betaine-sparing effects when added to practical poultry and swine feeds.
In summary, the present invention defines the role of thetins as specific
substrates for betaine-homocysteine methyltransferase that can enhance the
conversion of homocysteine to methionine, and as a result, be an effective
plasma homocysteine-lowering treatment for animals in need of such
treatment. The invention further defines the role of thetins as
methionine-, choline-, and betaine-sparing compounds that can be added to
animal feeds to reduce the costly addition of these nutrients to feeds
while maintaining optimal animal performance.
The following references were cited herein:
1. Koop, C. E., Coronary Heart Disease. In: Surgeon General's Report on
Nutrition and Health. PHS Publication number 017-001-00465-1, (1988).
2. American Heart Association, 1992 Heart and Stroke Facts. Dallas, Tex:
AHA, (1991).
3. Kang et al., Annu. Rev. Nutr. 12: 270, (1992).
4. Ueland et al., Plasma homocysteine and cardiovascular disease. In:
Francis RB, ed. Atherosclerotic cardiovascular disease, hemostasis and
endothelial function. N.Y.: Marcel Dekker, Inc., 183-236, (1992).
5. Motulsky, A. G., Am. J. Hum. Genet. 58:17, (1996).
6. Malinow, MR., Clin Chem. 40:173, (1994).
7. Nygard et al., J. Amer. Med. Assoc. 274:1526, (1996).
8. National Research Council, Nutrient requirements of poultry, 9th rev.
ed. Nat. Acad. Sci., Washington, D.C., (1994).
9. National Research Council, Nutrient requirements of laboratory animals,
4th rev. ed. Nat. Acad. Sci., Washington, D.C., (1995).
10. National Research Council, Nutrient requirements of swine, 9th rev. ed.
Nat. Acad. Sci., Washington, D.C., (1994).
11. Mitchell et al., J. Nutr. 108:67, (1978).
12. Steele et al., J. Biol. Chem. 253:7844, (1978).
13. Mudd et al., Metabolism 24:721, (1975).
14. Finkelstein et al., J. Biol. Chem. 261:1582, (1986).
15. Finkelstein et al., J. Biol. Chem. 259:9508, (1984).
16. Finkelstein et al., Biochem. Biophys Res. Comm. 66:81, (1975).
17. Jencks et al., J. Biol. Chem. 262:2485, (1987).
18. Yamamoto et al., J. Nutr. Sci. Vit. 41:197, (1995).
19. Harper et a., Amer. J. Physiol. 184:457, (1956).
20. Finkelstein et al., Arch. Biochem. Biophys. 146:84, (1971).
21. Wileken et al., New Eng. J. Med. 309:448, (1983).
22. Gahl et al., J. Inher. Metab. Dis. 11:291, (1988).
23. Wendel et al., Eur. J. Pediatr. 142:147, (1984).
24. Holme et al., Arch. Dis. Child. 64:1061, (1989).
25. Maw GA., Biochem J. 72:602, (1960).
26. Ericson LE., Acta Chemica Scand. 14:2101, (1960).
27. McKeever et al., Clinical Science 81:551, (1991).
28. Fromm et al., Arch. Biochem. Biophys. 81:363, (1959).
29. Lee et al., Arch. Biochem. Biophys. 292:77, (1992).
30. Durell et al., Biochim. et Biophys. Acta 26:270, (1957).
31. Sciba et al., J. Biol. Chem. 257:14944, (1982).
32. Ericson LE., Acta Chem. Scand. 14:2113, (1960).
33. Finkelstein et al., Biochem. Biophys. Res. Comm. 108:344, (1982).
34. Finkelstein et al., J. Nutr. 113:519, (1983).
35. Allen et al., Metabolism 42:1448, (1993).
36. Ericson LE., Acta Chem. Scand. 14:2127-2134, (1960).
37. Awad et al., J. Biol. Chem. 258:12790, 1983.
38. Finkelstein et al., Arch. Biochem. Biophys. 153:320-324, (1972).
39. Speed et al., J. Chromatog. 35:497, (1968).
40. Nelson et al., J. Chromatog. 324:203, (1985).
41. Ferber et al., J. Biol. Chem. 185:53, (1950).
42. Ip C, et al., Carcinogenesis 13:1167, (1992).
43. Finkelstein et al., J. Biol. Chem. 242:873, (1967).
44. Hanes C. S., Biochem. J. 26:1406, (1932).
45. Garrow TA, et al., FASEB J. 9:748, (1995).
46. Fromm HJ., Mol. Biol. Biochem. Biophys. 22:83, (1975).
47. Wileken et al., New Engl. J. Med. 309:448, (1983).
48. Laryea et al., Clin. Chim. Acta 230:169, (1994).
49. Du Vigneaud et al., J. Biol. Chem. 131:57, (1939).
50. Maw et al., J. Biol. Chem. 176:1037, (1948).
51. Maw GA., Biochem. J. 55:42, (1953).
52. Benevenga et al., Ann. Rev. Nutr. 4:157, (1984).
53. Patti et al., J. Nat. Prod. 56:432, (1993).
54. Rhodes et al., Ann. Rev. Plant Physiol. Plant Mol. Biol. 44:357,
(1993).
55. Ackman et al., J. Fish. Res. Bd. Canada 24:457, (1967).
56. Ackman et al., J. Fish. Res. Bd. Canada 29:1085, (1972).
57. Iida et al., Nippon Suisan Gakkaishi 52:557, (1986).
Any patents or publications mentioned in this specification are indicative
of the levels of those skilled in the art to which the invention pertains.
These patents and publications are herein incorporated by reference to the
same extent as if each individual publication was specifically and
individually indicated to be incorporated by reference.
One skilled in the art will readily appreciate that the present invention
is well adapted to carry out the objects and obtain the ends and
advantages mentioned, as well as those inherent therein. The present
examples along with the methods, procedures, treatments, molecules, and
specific compounds described herein are presently representative of
preferred embodiments, are exemplary, and are not intended as limitations
on the scope of the invention. Changes therein and other uses will occur
to those skilled in the art which are encompassed within the spirit of the
invention as defined by the scope of the claims.
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